1. Field of the Invention
The present invention relates to an exposure control system for photographic apparatus, in general, and to a method and appartus for compensating an exposure for variations in the response time of a shutter blade mechanism in such apparatus, in particular.
2. Description of the Prior Art
One well-known exposure control mechanism for a photographic camera is a so-called scanning blade shutter comprising two cooperatively moving shutter blade elements connected for opposed reciprocating movement with respect to each other. The shutter blade elements are formed with selectively shaped aperture openings which, depending upon the positions of the blades, symmetrically overlap over a light entry exposure opening within the camera housing. The blade elements are connected to define a progression of apertures over the light entry opening in the camera as a result of their connection with a so-called "walking beam" which is rotatably mounted with respect to the camera housing. When the "walking beam" is actuated by a combination spring and electrical solenoid coupled thereto, the shutter blade elements move simultaneously and in correspondence with each other to define a symmetrically configured and variable aperture opening over the camera light entry exposure opening.
Means are provided for automatically controlling the shutter blade elements to produce an exposure interval. They include the above-mentioned electrical solenoid and spring combination for actuating the shutter blade elements and, more particularly, the apertures formed therein, between opened and closed positions. In addition, means are provided for detecting and intergrating scene light in correspondence with scene light admitted by the apertures in the blade elements to the film plan during an exposure interval.
In a scanning blade exposure control system of the type described above, a force generated by the spring activates the shutter blade elements to produce a particular scene light admitting aperture size as determined, in part, by the above-mentioned scene light detecting and integrating means. When the required amount of scene light has been detected and integrated, a signal responsive thereto causes the electrical solenoid, which is coupled to each shutter blade element, to reverse shutter blade movement and reduce the particular light admitting aperture mentioned above, to zero. In order to produce the correct exposure, the light admitting aperture should theoretically reduce the particular aperture to zero as soon as it is commanded to do so. For any number of reasons this type of response will not occur without some additional control being provided.
One reason that a particular exposure aperture might grow larger than commanded by the exposure control system may be due to the unpredictable variations in the forces provided by the electrical solenoid and/or the spring that actuates the shutter blade elements between their opened and closed positions. Due to manufacturing tolerances, it is difficult to consistently produce a spring that will impart the same opening force to a pair of shutter blade elements. The force generated by one spring may be substantially greater or less than that generated by another such spring thereby causing the shutter blade elements to open at a faster or slower rate than another similarly actuated set of shutter blade elements. This variation in the speed of shutter blade elements movement can produce a substantially over or under exposed photographic image.
The same sort of shutter blade element speed variation can result from manufacturing tolerances between different electrical solenoids employed to actuate a pair of shutter blade elements to their fully closed positions. In the manufacturing process it is difficult to consistently wind a large number of solenoid coils having the same force-generating characteristics. Also, these solenoid coils are normally powered by a conventional battery whose output power is progressively reduced over its useful life. This reduction in power will reduce the force imparted to the shutter blade elements which will also produce an exposure affecting change in shutter blade element speed.
Factors such as friction between adjacent shutter blade elements and other movable members coupled thereto can also produce a variation in shutter blade element speed. Means for compensating for these factors have been disclosed in several forms such as the anticipation built into the photometer apertures found in some shutter blade elements through which the photometer senses scene light for the control of shutter blade elements movement. A problem with this and many other such solutions is that they are all incorrectly based on the assumption that the speed of the shutter blade elements remains constant from one exposure interval to the next. This assumption breaks down in practice due to the above-noted manufacturing tolerances in the shutter components and variations in friction due to change in the orientation of the camera by a camera operator during a normal series of picture taking operations.